Cleavers. 1 1 1 



stances is exceedingly fierce. For example, in water 

 only very small quantities of gas are dissolved, so 

 that all submerged water-plants have extremely thin 

 waving filaments instead of flat blades ; and one such 

 plant, the water-crowfoot, has even two types of 

 foliage on the same stem— submerged leaves of this 

 lacelike character, together with large, expanded, 

 floating leaves which loll upon the surface sonaething 

 like those of the water-lily. In the same way hedge- 

 row weeds, which jostle thickly against one another, 

 have a constant hard struggle for the carbon and the 

 sunshine, and grow out accordingly into numerous 

 ^mall subdivided leaflets, often split up time after 

 time into segments and sub-segments of the most 

 intricate sort. I do not mean, of course, that each 

 individual leaf has its shape wholly determined for it 

 by the amount of sun and air which it in particular 

 happens to obtain, but that each species has slowly 

 acquired by natural selection the kind of leaf which 

 best fitted its peculiar habitat. Those plants survive 

 whose foliage adapts them to live in the circum- 

 stances where it has pleased nature to place them, 

 and those plants die out without descendants whose 

 constitution fails in any respect to square with that 

 inconvenient conglomeration of external facts that 

 we call their environment. 



